Olefins (CnH2n), such as ethylene (C2H4), are produced by pyrolysis of the hydrocarbons (naphtha, natural gas, ethane, or the like). In particular, to produce olefinic hydrocarbons (ethylene, propylene, or the like), the hydrocarbons are supplied with steam into a tube provided in a reacting furnace and heat is supplied to the hydrocarbons from outer surface of the tube so that a pyrolysis reaction of the hydrocarbons can be generated in the tube. The tube is made of a high-Cr and high-Ni alloy as represented by a 25% Cr-25% Ni alloy, a 25% Cr-38% Ni alloy or the like, or is made of a stainless steel as represented by AISI 304 type.
In the above described pyrolysis reaction, it is necessary to transfer heat (which is supplied to the outer surface of the tube) from the outer surface of the tube to the inner surface of the tube efficiently, so as to prevent unreacted hydrocarbons from discharging to outside of the furnace. That is, the tube needs superior “heat exchange characteristic”. The heat exchange characteristic can be evaluated by measuring the average temperature of fluid at the outlet of the tube. When the tube has the superior heat exchange characteristic, the average temperature of the fluid at the outlet of the tube is increased.
A mixed gas composed of hydrocarbons and steam is supplied into a steel tube from an inlet of the steel tube with low pressure and high speed. Unreacted mixed gas and newly formed gas due to the reaction move a long distance along the ribs provided on the inner surface of the tube. Therefore, the gas flow is interrupted depending on the shape of the ribs. In this case, a fluid in the central portion of the tube and a fluid in the bottom part of the rib are separated. Thus, a mass transfer (reaction) between the central portion of the tube and the bottom part of the rib becomes insufficient. In such cases, since the reaction products are accumulated in the bottom parts of the ribs, an over pyrolysis reaction occurs. On the contrary, the reaction becomes insufficient in the center portion of the tube, leading to the yield loss. In order to solve such problems, the tube should have superior “pyrolysis reaction characteristics”. The reaction characteristics can be evaluated by the deviation of temperatures at outlet of the tube, since the pyrolysis reaction characteristics depend on the mass flow in the tube.
Patent Document 1 (JP 58-173022A) discloses a production process of a tube with intratubular spiral ribs. In the above production process, the tube with intratubular spiral ribs is produced by torsional work from a metal tube with intratubular straight ribs which is produced by hot extrusion processing. Patent Document 2 (JP 01-127896A) discloses a tube material for a heat exchanger with wavy shape on the inner surface of the cross section, in which the radius of convex curvature of the crests, RF, and the radius of concave curvature of the valleys, RS, satisfy a relationship of RS≧RF.
Furthermore, Patent Document 3 (JP 08-82494A) discloses a tube for heat exchange. The tube is provided with fins which are formed on the inner surface of the tube at given pitches and extending to directions which intersect with the tube axis. In particular, the fins are arranged in one or a plurality of areas on the inner surface along the tube axis direction or in an entire area on the inner surface. Patent Document 4 (JP2005-533917A) discloses a tube with intratubular spiral fins, which is used for the pyrolysis reaction process of hydrocarbons under the existing steam.
The tubes with intratubular ribs or fins disclosed in the above described Patent Documents, however, cannot balance the “heat exchange characteristic” and the “pyrolysis reaction characteristic”, and cannot improve both of the characteristics sufficiently. Thus, heat exchangeable tubes with intratubular ribs, in which both of the characteristics described above are further improved, were desired.
Meanwhile, regarding the usage conditions of metal tubes used in the pyrolysis reaction in a cracking furnace of an ethylene plant or the like, the temperature tends to become higher due to yield improvement, with a recent increase of resinoid demand. In such metal tubes used for pyrolysis reaction at higher temperatures, carbon is unavoidably formed due to the pyrolysis reaction. Then, the carbon is attached to the inner surface of the tube and deposits on the inner surface. This phenomenon is called “coking”.
When coking occurs, the pyrolysis reaction efficiency decreases since the deposited carbon prevents transferring the heat supplied from the outer surface of the tube to the mixed gas. Furthermore, the steel tube becomes brittle since the accumulated carbon diffuses inside the steel tube which causes carburization of the steel tube. Thus, the damage of the steel tube is caused from the carburization portion. Moreover, when the carbon, which is flaked from the deposited layer, accumulates in the steel tube, the gas flow is interrupted and the pyrolysis reaction is inhibited as well and causes the above described damage. In addition, when the carbon deposits in large amounts, serious accidents such as an explosion or the like may take place. Therefore, a periodically flowing air and steam into the tube so as to oxidize and remove the precipitation carbon i.e. decoking is carried out in a practice. However, the decoking work leads to big problems such as a shutdown during the decoking work and an increment of man-hour or the like.
The inner surface of the metal tube for pyrolysis reaction is exposed to the carburizing gas atmosphere containing hydro carbon gas, CO gas, or the like. Therefore, a heat resistant material having resistances to carburization and coking in the carburizing gas atmosphere is required as a tube material.
Patent Document 5 (JP 2005-48284A) discloses a stainless steel tube that consists of a mother material including 20 to 35 mass % Cr and the tube has resistances to carburization and coking. The disclosed tube has a surface layer comprised of a Cr-depleted layer that includes more than or equal to 10 mass % Cr and which thickness is less than or equal to 20 μm. Although it is disclosed that protrusion, fin or the like may be provided on the inner surface of the tube in the Patent Document 5, the specific configurations are not disclosed at all.
[Patent Document 1] JP 58-173022A
[Patent Document 2] JP 01-127896A
[Patent Document 3] JP 08-82494A
[Patent Document 4] JP2005-533917A
[Patent Document 5] JP 2005-48284A